Command line options:
http://hostname:9981/
By default everyone (also from remote hosts)is allowed full access to all features / settings in the web user interface. If this is the first time you setup Tvheadend you are most encouraged to enter the web user interface, selected the 'Configuration' + 'Access Control' tab and make reasonable changes. Further help / documentation can be obtained inside the web interface.
If Tvheadend is started without the '-f' argument it will not demonize nor change its primary UID/GID. Rather it will run with the permissions granted to the user executing the binary. If that's the case you must make sure the current user is granted with access to the video devices.